How long do I need to keep 400k in the bank for my NON-O marriage extension in Thailand?

The actual rule of the police order regarding marriage extension says the funds only need to stay in the account until you apply for the extension.

But with that being said, many immigration offices want you to keep the funds in until you go back and get the yearly extension inked in.

Once you have the extension you're fine and you can pull the money out.

I'd say seeing as you're under consideration. Is soon to be over you might as well let the money in the account until you get the year extension